Plucked straight out of a 12-second ’65 Mustang coupe, this 347 stroker perfectly represen
Post Dyno Analysis
What started out as a simple cam test turned out to be a real head-scratcher, as the results were puzzling for a number of reasons. First off, the biggest cam didn’t make the most power. Secondly, the Thumpr’s specs suggest that it should run out of breath the earliest. Instead, it proved to be the most rev-happy cam of the bunch. The most obvious distinguishing factor between the Thumpr and its test mates is its tight LSA. Whereas Cam A and Cam B both shared 112-degree LSAs, the Thumpr was ground at 107 degrees. Tighter lobe-separation angles generally boost low-end torque at the expense of top end power. Furthermore, compared to Cam B, the Thumpr has an earlier intake valve closing point of 65 degrees after bottom dead center as opposed to 70 degrees ABDC. Intake valve closing is by far the most important of the four valve events in terms of power output, and a later IC generally improves top end power while sacrificing low-end torque. Again, neither of these scenarios held true during our test.
Seeking the expertise of someone much smarter than the typical journalist, we ran the results by SAM’s Head Instructor Judson Massingill for a more educated opinion. As it turns out, it’s all about cylinder pressure. “The intake duration of the Thumpr cam falls right in between Cam A and Cam B, so duration isn’t what’s accounting for the power difference. Plus, the Thumpr has the least amount of lift, so you can throw that out of the equation as well,” he says. “What’s going on in this motor is that the tighter LSA and increased overlap of the Thumpr is helping pull more air through the intake valve. The improvements in intake charge filling and cylinder pressure is what’s bumping up both torque and horsepower. Tight LSA cams usually drop off at high rpm, but the reason that’s not happening in this instance has to do with cylinder pressure. On a relatively low compression motor like this 347, you don’t have much cylinder pressure to begin with, so having a wide LSA really hurts you. With a tighter 107 LSA, the increase in cylinder pressure helps make up for the low compression ratio of the motor. If this 347 had a higher compression ratio, the tighter LSA of the Thumpr would have made less of a difference on the dyno.”
Formerly occupying the cam bores in the 347 was a mild hydraulic flat-tappet grind that fe
Judson says, what makes this best-of-both-worlds scenario possible, in which low-end torque doesn’t come at the expense of top end power, are cylinder heads that flat-out get the job done. “When you have heads that move a lot of air, you can get by with a short-duration camshaft because tightening up the LSA increases cylinder pressure everywhere. You always want the best heads possible with the least amount of cam as possible,” he says. “In an application like this 347, the only reason to run a wider LSA is to smooth out the idle quality. As long as you can get it to hook, the Thumpr would be at least two-tenths quicker at the track than the other cams in this test. Not only does it produce more midrange power, it also carries that power to a higher rpm.”
Ultimately, no amount of speculating is an adequate substitute for good old-fashioned dyno testing. Since speculation doesn’t always hold up on the dyno, it’s not a bad idea to dyno test multiple cams in your engine combo of choice before deciding on the perfect grind for your application. Compared to what it costs to build a motor, cams are relatively inexpensive, so it’s cheap experimentation.
Although hot rodders are penny pinchers by nature, from a tuning perspective, the $500 or
Before pulling the handle, SAM students set the ignition timing to 34 degrees of advance.
Fueling the 347 was a 4150-series Holley 750-cfm carb. Pulling the spark plugs after the f
While they probably hurt peak power numbers a bit, we dyno tested the 347 with the same 1⅝
The first series of dyno pulls with the 224/232-at-.050 cam (the small Cam “A”) sent the 3
Granted, they didn’t contribute to the horsepower tally one bit, but COMP’s slick cast-alu